The present invention relates generally to internal combustion engines for automotive vehicles, and more specifically, to an automotive vehicle having a starter/alternator coupled to the engine.
Automotive vehicles with internal combustion engines are typically provided with both a starter motor and alternator. In recent years, a combined alternator and starter motor has been proposed. During initial startup of the vehicle, the starter/alternator functions as a starter. While functioning as a starter, the starter/alternator provides a sufficient amount of torque to rotate the crankshaft of the engine before the cylinders are fired.
After the engine is started, the starter/alternator is used as a generator to provide electric power to the electrical system of the vehicle and/or for driveline damping.
In foreseeable automotive applications, the engine may be shut down during stops (e.g., red lights). When the accelerator is depressed the engine will resume firing. Thus, many startups would occur over the course of a trip. Acceleration in such systems is inherently low absent intervention since torque levels from the engine upon startup are low. Thus, starter/alternators may be used to provide boost torque to help accelerate the vehicle as well.
As shown in FIG. 1, there are two distinct features that characterize the torque versus speed requirement of starter/alternators. The first is the high torque required to provide cold engine starts. The second distinct feature is the high generating power requirement in a wide speed range. To meet both the starting torque and generating power requirements simultaneously, the stator windings of a starter/alternator are typically designed with a low number of series turns to allow for a wider generating speed range. In order to simultaneously meet the starting torque requirement with a low number of series turns a high current level is needed, resulting in the starter/alternator having a relatively high peak current rating which often exceeds 700 amps for a 42-volt electrical system. The high peak current increases the current rating for the power devices, cabling and connectors. Also, cost, thermal requirements, electromagnetic interference and compatibility issues are also raised.
It would therefore be desirable to provide a starter/alternator capable of operating with a decreased peak current requirement during starting and which operates with an extended speed range for generating.
Known methods for accomplishing these goals include reconfiguring the stator windings from series to parallel or by changing them from a wye to a delta winding configuration. These types of configurations allow the stator winding to have a greater number of series turns during starting than during generating. Typically, however, such systems are very complex and costly due to the mechanical switches needed to provide the reconfiguration.
Another known method for achieving these goals is to provide a DC to DC boost converter to supply the inverter and the machine with an increased voltage during the generating mode over that of the nominal battery power provided in the vehicle. This allows the number of series turns in the stator windings to be increased, which decreases the amount of current needed to achieve the same starting torque. The cost associated with such a DC to DC converter is typically prohibitive.
It would therefore also be desirable to avoid the above-mentioned problems of reconfiguring the stator windings or adding a DC to DC converter to allow high starting torque and a wide speed range.
It is therefore one object of the invention to provide an improved integrated starter/alternator system suitable for use as a starter motor that allows high torque to be achieved during starting while providing a high generating power requirement over a wide range of speeds.
In one aspect of the invention, the system comprises an inverter circuit, a rectifier circuit and an electric machine, which has a stator circuit coupling to the inverter circuit and the rectifier circuit. A pair of switches selectively couples the rectifier circuit to the inverter circuit so that in a startup mode the switches are closed and in a generator mode the switches are open.
In a further aspect of the invention, a method of operating a system having a power supply, an electric machine with a stator winding circuit, an inverter circuit and a rectifier circuit comprises the steps of:
in a starting mode of operation, directly coupling the power supply to the inverter circuit though a pair of switches;
in a generating mode of operation, opening the pair of switches to decouple the inverter circuit from the power supply.
One advantage of the invention is that the current capacity required for the wiring and connectors is substantially reduced. This is true even though a higher voltage requirement exists for such configurations.
Another advantage of the invention is that the relatively large number of series turns connected to the inverter portion of the controller is such that the amount of magnetizing current is lower when compared with a lower turn number machine design. This reduces the inverter switching and conduction losses. Yet another advantage of the invention is that the turns ratio between the two stator coil groups may be changed depending on the particular system requirements.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.